The present invention relates to a method for recovering petroleum from deposits which have already been worked by primary extraction or are not suitable for primary extraction.
Petroleum deposits comprise reservoir rock formations which are covered on top by impermeable layers. The oil is contained in the reservoir rock formations and different binding forces act between the oil and the rock. Petroleum is recovered from minable deposits by primary extraction processes. Particularly important primary extraction processes are the deep well extraction process and the gas lift extraction process.
Only about one-third of the petroleum quantities existing in the deposits can be recovered by the primary extraction. When the oil yield decreases in primary extraction, so-called secondary or tertiary extraction processes are used to increase the degree of oil removed from the deposit. The secondary or tertiary extraction processes are also used in deposits which, for economic reasons, cannot be worked with primary extraction methods (e.g. tar sands).
The secondary extraction processes can be divided into three groups. The first group comprises processes which influence the pore content of the reservoir rock formations by introducing foreign energy. Exemplary of such processes are the gas injection process and the vacuum process. The second group comprises processes which increase the surface area of the pore system of the reservoir rock. Exemplary processes of the second group include processes which employ acidification and those which employ hydraulic crack formation. The third group comprises processes which change the physical forces between pore content and pore wall in the reservoir rock. Exemplary processes of the third group are those which employ surface active substances, bacterial activity, or heat.
German Auslegeschrift DE-AS No. 1,493,190 discloses a method for separating mixtures of organic substances by treating the mixtures of organic substances with supercritical gas and subsequently separating the substances dissolved in the resulting supercritical gas phase by reduction of pressure and/or increase in temperature. This publication does not contain any disclosure relating to recovering petroleum from deposits which have already been worked by primary extraction or are not suitable for primary extraction by using supercritical gases.